Airplanes often carry with them other flying vehicles, such as smaller airplanes or missiles, which are to be launched during flight. The second vehicle typically is located on the wing of the first vehicle. Both vehicles have inertial measurement units (IMUs) on them for determining their inertial locations.
In order to operate, IMUs require to know the initial position, velocity and attitude of the vehicle with respect to some predefined coordinate system.
During flight, the navigation system of the main vehicle continually operates to determine the attitude, velocity and position of the vehicle. Before the second vehicle is launched, the main vehicle provides the initial conditions to the IMUs of the second vehicle. As long as the exact position, velocity and attitude of the second vehicle with respect to the main vehicle are known and as long as the current values are accurate, the second vehicle will receive an accurate set of initial conditions.
However, the output of the IMU on the second vehicle tends to drift (i.e. lose accuracy) over time and, more importantly, due to vibrations in flight, the second vehicle might rotate from its nominal position. If the extent of the rotation is not compensated, the IMU output of the second vehicle will not be reliable.
The rotation can be estimated by performing a maneuver which excites lateral acceleration. The output of both sets of IMUs are compared and the angle of rotation of the second vehicle vis-a-vis the main vehicle is determined.
Pitch and roll angles are not difficult to estimate. However, the standard maneuver for yaw estimation, illustrated in FIG. 1 to which reference is now made, requires curving in and out along a curve 12, horizontal to the ground 10. Pilots generally do not like to perform such a maneuver just prior to releasing the second vehicle. However, without it, the navigation system of the second vehicle is not properly calibrated.